In the early days of the automobile, naphtha was recovered by simple fractionation from crude oil to produce gasoline. This was an adequate fuel for the low compression engines in use then, but as compression ratios increased there were demands for better fuel.
Thermal, and then catalytic, cracking produced gasoline of higher octane number. To meet demands for more octane refiners added lead compounds to gasoline, and/or installed alkylation units (using the light olefins generated by the cracking units) and catalytic reformers using platinum based catalyst.
The quality of the fuel increased, but so did the cost and complexity of the refinery. These complex refineries made a mix of clean fuels and fuels with relatively large amounts of sulfur and nitrogen or benzene.
Catalytic reforming produced a benzene rich gasoline of superior quality and low sulfur and nitrogen levels. Feed hydrotreating was required. Fortunately the reformer generally made enough hydrogen to supply the demands of its hydrotreater.
Alkylation produced excellent gasoline that was also a clean fuel. Alkylate was high in octane, and exceedingly low in sulfur, nitrogen and benzene because it was made from clean light hydrocarbons. Most of the feed to the alkylation unit was a byproduct of the catalytic cracking process. A clean fuel (alkylate) was made in parallel with large volumes of high octane catalytically cracked FCC naphtha, with a high sulfur and nitrogen content.
The demand for cleaner fuels has been difficult to cope with in many refineries. Refinery processes produce gasolines which are extraordinarily clean (alkylate and reformate) and which contain many impurities (FCC naphtha). Blending was a way to make moderately clean fuels, but could not be used to produce an adequate amount of reformulated gasoline.
Hydroprocessing of feed and product streams can help, but FCC naphtha will still have some sulfur and nitrogen in it. Such processing is expensive and requires hydrogen, which is not available in some refineries.
Refiners need more clean fuels, and in many instances even specific types of clean fuels, such as oxygenates. To make these materials olefins are usually needed, and there is no easy way to increase production of olefins. While processes and catalysts are known which can convert paraffins into olefins, these generally suffer from one or more of the following deficiencies:
Short catalyst life (if operated without H.sub.2 addition).
Low conversion (if operated to maximize catalyst life). PA1 High capital and operating expense (if operated with large amounts of hydrogen to extend catalyst life, or run at high conversion levels).
To summarize, most refineries do not have hydrogen to spare, and would like more olefins. Some refineries wish to run their dehydrocyclization units more efficiently, but without spending too much on recycle gas compression.
Some art on conversion of light paraffinic streams into olefins and/or aromatics will now be reviewed.
Some dehydrogenation catalysts, such as Pt-In-zeolite can be used without H.sub.2 addition, but catalyst life is short unless large amounts of H.sub.2 are added to the feed. Some of the extensive art on dehydrogenation technology will be reviewed.
U.S. Pat. No. 4,886,926 Dec. 12, 1989, Dessau, which is incorporated by reference, discloses making unsaturates from a paraffinic C.sub.2 -C.sub.5 stream over catalyst of 0.01 to 30% Group VIII metal, preferably Pt, and non-acidic crystalline material containing 0.05 to 20% tin. The crystalline material is selected from the group of ZSM-5, ZSM-11, ZSM-12, ZSM-20, ZSM-23, ZSM-48, ZSM-50, MCM-22.
U.S. Pat. No. 4,935,566 Dessau, et al (Jun. 19, 1990), incorporated by reference, discloses dehydrocyclization and reforming paraffins over a non acidic platinum-tin containing crystalline micro porous material.
U.S. Pat. No. 5,192,728 Mar. 9, 1993, Dessau, which is incorporated by reference discloses a catalyst of a dehydrogenation metal and non-acidic crystalline material and tin.
U.S. Pat. No. 4,990,710, which is incorporated by reference, disclosed catalytic dehydrogenation using a low acidity dehydrogenation catalyst. Reactor "inlet H.sub.2 /feed ratios are 5 or less; even at reactor inlet ratios of zero (0), there will be a hydrogen partial pressure in the reactor because hydrogen is a bi-product of dehydrogenation." Col. 7 lines 16-29.
'710 summarizes the state of the art in dehydrogenation. The process can operate with no H.sub.2 added, but adding, e.g., up to 5:1 H.sub.2 :HC molar basis makes the catalyst last longer.
Refiners wishing to catalytically convert paraffins into olefins or into aromatics are left with difficult choices. They can add lots of hydrogen to achieve a satisfactory catalyst life, and pay more for equipment to deal with the added vapor, which may include a hydrogen recycle gas system or regenerate catalyst frequently to remove the coke which rapidly forms. They also will see an adverse affect on equilibrium and hence conversion. Hydrogen addition to a hydrogen producing process suppresses to some extent hydrogen production. The alternative, operating with no hydrogen addition, results in drastically shortened catalyst life due to rapid coke buildup on the catalyst.
I discovered a way to have the best of both approaches. By operating my endothermic reactor(s), with an unusual approach to pressure drop, it was possible to obtain most of the catalyst life associated with high pressure operation while achieving most of the yield advantages associated with low pressure operation.
The first reactor or reaction zone runs at relatively high pressure, preferably with a modest amount of hydrogen present. Such high pressure operation gives relatively poor yields of hydrogen, but the catalyst exhibits great stability at such pressures. The hydrogen production from the first reactor provides a hydrogen rich atmosphere, which reduces coke formation in downstream reactors, even when the pressure is reduced to promote increased dehydrogenation or dehydrocyclization.
By operating this way it is possible to greatly reduce or perhaps even eliminate most or all of the capital and operating expenses associated with conventional hydrogen addition techniques. There is much less demand on the recycle gas compressor, even if one is used. It also increases the effective capacity of the heaters or heat exchangers used to preheat feed to the first reactor. A key benefit is reduction of the hydrogen partial pressure at the outlet of the last stage of the dehydrogenation reactor which allows maximum conversion of the paraffins.